Portable information terminals such as mobile phones and laptop personal computers have been rapidly reduced in size and weight in recent years. In line with this trend, non-aqueous electrolyte secondary cells such as lithium-ion secondary cells have been widely used, because of their high energy density and high capacity, as power sources for driving the portable information terminals. These non-aqueous electrolyte secondary cells are also being increasingly used as power sources for driving such devices as electric vehicles (EVs) and hybrid electric vehicles (HEVs) that require a large amount of current.
The non-aqueous electrolyte secondary cells include a negative electrode made of carbon materials. Of the materials, graphite-based carbon materials such as natural and artificial graphite are more used, because of their larger discharge capacity, than amorphous carbon materials.
The cells used as the power sources for driving EVs and HEVs are required to have high-rate charge-discharge capacity characteristics suitable to perform rapid charge and high load discharge. In a cell including a graphite negative electrode, however, high-rate charge-discharge operations often cause lithium deposition on the surface of the graphite negative electrode. The deposited lithium does not contribute to the subsequent charge-discharge cycles, resulting in a decrease in discharge capacity (remaining capacity) after repeated cycles.
In this situation, Patent Documents 1 to 6 present the use of various negative electrode active materials prepared by mixing different kinds of carbon materials.